A head-up display (HUD) is an electronic display implemented in a fashion that allows a user to maintain focus in an area that the user was looking at, while still viewing the display. A HUD is implemented on a transparent display, and has conventionally been placed in planes.
In recent times, the HUD is implemented in a vehicle, and in other contexts. Thus, the HUD is become more and more robust and important in electronic control systems and display presentations.
In certain implementations, the HUD may be a stand-alone device, and placed on a surface, such as a dashboard, or hung from a car's roof. In another example, the HUD may be incorporated into a pre-existing window surface, for example a front or side window of a vehicle.
The basic operation of a HUD is that each pixel or portion of displayable space is configured to light up with a specific amount of luminance. The information being conveyed may be communicated by an electronic driver, and attached to various electronic systems implemented along with the HUD. For example, in response to the HUD being implemented in a vehicle, the HUD may be configured to display the speed of the vehicle.
However, because light is employed to convey the electronic information, the viewing experience is affected by the environment in which the HUD is implemented in. For example, if the electronic display is an awkward or inconvenient location, viewing the electronic display may be ultimately frustrated.
Further, the environment around the electronic display may be dynamic and changing. For example, if the electronic display is implemented in an area that interacts with outside or external light providing sources, the HUD's ability to convey information via the lighted elements may be obstructed or modified.
A measure of unit for determining the intensity of light being transmitted or propagated in a specific direction is known as luminance. Various units may be employed to measure luminance, such as a candela per square meter. One of ordinary skill in the art may appreciate that several units or types of measurements may be employed for luminance measurement.
For example, if the HUD is implemented in a vehicle, the electronic display may interact with the outside lighting environment. Thus, several factors may be present with the exterior lighting to affect the display's ability to provide a clear display in an optimal fashion. For example, the exterior lighting may be affected by the cloud cover, the weather, the road (e.g. if the vehicle is in a tunnel), the time of day, or the like.
Thus, an electronic display may be aided greatly by an ability to be cognizant of the exterior lighting conditions. Based on the knowledge of the exterior lighting conditions, the electronic display may adjust the display luminance accordingly.
One such example of a system for adjusting display luminance is shown in FIG. 1. FIG. 1 illustrates an example of a system 100 for adjusting display luminance according to a conventional implementation. Because the system 100 is known in the prior art, a detailed explanation will be omitted. System 100 is referred to as a linear light system. Linear light systems may not work over specific dynamic ranges, such as 6-8 decades. Over these dynamic ranges, an analog-to-digital converter may be inadequate.
FIG. 2 illustrates an example scenario in which light may affect a viewer of a HUD 200. The HUD 200 shown in FIG. 2, is implemented in a vehicle 250. The HUD 200 is gazed upon by a viewer 220. In response to light rays 210 being propagated from an exterior light source (for example, the sun or other light generating sources), the viewing of the HUD 200 via the viewer 220 may be obstructed and modified.
Employing the linear system 100 discussed above may be ineffective in counteracting the effects of the light rays 210 for at least the reasons discussed above. Thus, employing conventional techniques to adjust a HUD 200 may be ineffective and not very robust.
In order to understand how to adjust display luminance, the Silverstein relationship is provided (as explained in several references submitted). The equation described below describes a relationship between the detected DBL and the luminance to be employed in a display.ESL=BO(DBL)C                 the terms being defined as:        ESL=Emitted Symbol Luminance in cd/m2         BO=Luminance Offset Constant        DBL=Various Display Background Luminance in cd/m2         c=Power Constant (slope of the power function in logarithmic coordinates).        
With cathode ray tubes (CRT) display technologies, phosphor reflectance does not change as a function of phosphor light emission. A liquid crystal display (LCD) presents a different challenge due to the “on” and “off” state each LCD cell experiences. Thus, variations of the Silverstein relationship may be calculated for LCD displays and HUD based technologies. However, by employing the DBL relationship above, the display visibility may be greatly improved.
However, non-linear forward looking light sensors have not been implemented in existing HUD technologies, and the light sensors that have been employed (integrating the Silverstein methodology discussed above), are linear.